The present invention generally relates to a method and apparatus for compensating for DC-offset when receiving signals in a radio receiver. More specifically, the present invention proposes a method and apparatus for compensating for DC-offset introduced in the radio receiver in such a way that the DC-offset estimation and channel estimation are separated, and that any bias in the DC-offset estimation due to the transmitted symbols is compensated for in a channel estimator and in an equalizer.
In digital communications systems, transmission signals are produced by the modulation of a carrier signal with digital data to be transmitted. The digital data is commonly transmitted in bursts where each burst consists of a number of data bits. When the transmitted signal is received, the signal requires demodulation in order to recover the data.
Radio receiver architectures commonly employ direct conversion (i.e., homodyne) receivers to perform the demodulation of a received signal. A local oscillator operating at the carrier frequency is used to mix down the received signal to produce in-phase (I) and quadrature (Q) baseband signals. Direct conversion receivers are very efficient in terms of both cost and current consumption. The motivation behind the direct conversion receiver is to have the incoming carrier directly converted down to baseband, in both I and Q components, without use of any IF frequencies. However, direct conversion receivers also have drawbacks. For example, a DC-offset can be introduced to the DC level of received signal. A DC-offset arises from mainly three sources: (1) transistor mismatch in the signal path, (2) local oscillator signal leaking and self-downconverting to DC through the mixer, and (3) a large near-channel interferer leaking into the local oscillator and self-downconverting to DC. As a result, a signal that is received from a transmitter can be farther distorted, and thereby lead to inaccurate data decoding. Additionally, the DC-offset can be several decibels (dB) larger than the information signal, requiring the DC-offset to be compensated for in order to be able to recover the transmitted data in the decoder.
The simple and most immediate way to compensate for the DC-offset is to estimate the mean value of the received burst, subtract the estimate from the received signal and then feed the signal to the decoder. However, the estimate introduces a bias DC offset, due to the finite amount of data used for estimating the DC-offset. The bias DC offset can be so large that the bit error rate of the receiver does not decrease as the signal-to-noise ratio increases. As a result, the bias DC offset will determine the minimum amount of noise (i.e., the noise floor) that is combined with the data within the receiver.
Furthermore, since the transmitted data is unknown, it is impossible to compensate for the bias DC offset in the signal before it is supplied to the decoder unless a large amount of data is received (in which the bias DC offset will be reduced to zero) or both the transmitted symbols and the channel are known. A way to overcome this problem is to compensate the DC level in the decoder. However, while this solves the bias DC offset problem, the dynamics in the decoder will be too large because the DC-offset level can be several decibels (dB) larger than the received signal. Also, numerical problems are encountered when estimating the radio channel and the DC-offset simultaneously because of the magnitude difference between the channel parameters and the DC component. Therefore, there is a need for methods and apparatuses that separate the mean value estimation and channel estimation tasks and that also compensate for the bias DC offset introduced by the transmitted sequence.
To remedy the problems encountered in conventional DC-offset compensation techniques, the present invention provides the ability to separate mean value estimation and channel estimation and the ability to compensate for bias DC offset introduced by the transmitted sequence.
In accordance with an exemplary embodiment of the present invention methods and apparatuses are disclosed that can compensate for DC-offset in a receiver by receiving a transmitted signal burst at the receiver; downconverting the signal burst into a set of baseband component values; finding a known training sequence in the set of baseband component values; estimating a DC-offset value using the known training sequence; subtracting the DC-offset value from the set of baseband component values to obtain a second set of baseband component values; performing channel estimation using the set of second baseband component values and outputting a channel model and a bias DC offset value, and performing equalization of the second set of baseband component values using the second set of baseband signals, the estimated channel model and the bias DC offset value.
Additionally, in accordance with another exemplary embodiment of the present invention methods and apparatuses are disclosed that can compensate for DC-offset in a receiver where the received DC level is not constant, by determining the location of at least one DC step value within the received signal and performing DC estimation based upon the known training sequence and the location of the at least DC step value one step value.